SECTION VIII.
FORAMINATED POLYTHALAMOUS SHELLS.
Nummulites.
If the present were a fit occasion for such minute inquiries, the investigations of the various known species of Microscopic shells would unfold a æries of contrivances having relation to the economy of the minute Cephalopods by which they were constructed, not less striking than those we have been examining in the shells of extinct Genera and species of larger Cephalopods. M. D'Orbigny has noticed from 600 to 700 species of these shells, and has prepared magnified models of 100 species, comprehending all the Genera.[1]
The greater number of these shells are microscopic, and swarm in the Mediterranean and Adriatic. Their fossil species abound chiefly in the Tertiary formations, and have hitherto been noticed principally in Italy. (See Soldani, as
- ↑ M. D'Orbigny, in his classification of the shells of Cephalopodous Mollusks, has established three orders. 1. Those that have but a single chamber, like the shell of the sepia and horny pen of the Loligo. 2. Polythalamous shells, which have a siphuncle passing through all the internal chambers, and which terminate in a large external chamber, beyond the last partition, such as Nautili, Ammonites, and Belemnites. 8. Polythalamous internal shells, which have no chamber beyond their last partition.
Shells of this order have no siphuncle, but the chambers communicate with each other by means of one or many small foramina. On this distinction he has founded his Order Foraminiferes, containing five families and fifty-two genera.
It maybe necessary to apprise the reader that doubts have been entertained as to the cephalopodous structure of some of these minute multilocular shells; and that there are not wanting those who attribute to them a different organization.
